Cell cultures of an NGF-responsive clonal cell line of rat pheochromocytoma cells (PC12) and primary cultures of rat and chick sympathetic neurons will be used as model systems for surface changes induced by peptide hormone and growth factor binding and internalization. The PC12 line resembles adrenal cromaffin cells and in the presence of nerve growth factor (NGF) expresses neuronal properties such as neurite outgrowth and electrical excitability. With initial exposure to NGF the surface of the PC12 cells undergoes a rapid sequential change in surface morphology. Ruffles and pits are prominent in these surface changes. The PC12 line is put forth as a useful model for the study of the mechanisms of action of peptide hormone binding and internalization and for the investigation of the cell surface during neuronal differentiaton.